Answer:
In DNA and RNA, guanine forms three hydrogen bonds with cytosine. Three bonds are formed between these bases as a result of the fact that guanine possesses two nitrogen and one oxygen atoms that can form hydrogen bonds. Given the spatial restrictions of the double helix, which call for there to be one purine and one pyrimidine in each base pair, A always pairs with T and G always pairs with C as these are the only combinations that for hydrogen bonding to take place.